The proposed experiments examine the effects of various intensities of light and the morphology and function of retinas of rats maintained in cyclic light condition, i.e. 12 hours light, 12 hours dark. The principal aim of these experiments is to establish non-damaging light intensity levels for rat maintenance facilities. Systematic studies in this regard have not been performed. These results will provide important information which may provide a data base from which the Committee on Care and Use of Laboratory Animals can recommend safe lighting conditions for animal maintenance facilities. we shall determine the minimum light intensity which produces irreversible functional and/or morphological damage in the retinas of albino rats maintained in a cyclic light environment over a period of time which approximates the usual tenure of rats in an animal facility (6 months). For each of the light intensities tested, groups of rats will be examined after various durations of light exposure. Histological and ERG recording procedures will be used to determine the morphological and functional integrity of the retina, respectively. We shall next determine the capacity of the retinal to revover from cyclic light-induced damage. The time course of morphological and physiological recovery will be monitored during a four week recovery period using historical and ERG recording procedures. Finally, we shall determine the effect of toxic levels of light on retina photoreceptor renewal processes. Autoradiographic analysis of the retina following an injection of 3H-amino acids will provide information concerning the viability of outer segment renewal processes in the damaged photoreceptor.